1. Field of the Invention
The present invention relates to a bipolar power transistor with high collector breakdown voltage and a corresponding manufacturing process.
2. Discussion of the Related Art
In bipolar power transistors fabricated by planar technology the collector region sustains almost the entire reverse voltage applied to the base-collector junction. This is a consequence of the electric field distribution across the junction, which depends on the doping concentration profile of the transistor's base region.
Because of this limitation, prior art, high voltage bipolar transistors do not use a planar structure. Rather they use a so-called "mesa" structure in which each transistor resembles a little mesa whose vertical walls terminate the base-collector junction.
One of the basic technologies by which mesa transistors are fabricated is known as MultiEpitaxial Mesa ("MEM"). In the MEM technology, the base region comprises a lightly doped P- epitaxial layer, grown over an N- epitaxial layer (from which comes the name of the technology). A P+ region is then diffused therein, see e.g., (article title), Solid State Technology, March 1976 at 29-32. The corresponding base doping concentration profile causes a significant percentage of the total reverse voltage applied to the base-collector junction to be sustained by the P- epitaxial portion of the base region, i.e., not the N- epitaxial collector only. The electric field distribution in both the base and the collector regions determines some of the electrical characteristics of the transistor, such as the gain dependence on the collector current and the ruggedness in secondary breakdown and in direct breakdown.
Even though this technology allows the fabrication of transistors with collector breakdown voltages up to 2000 V, it involves the passivation of the base-collector junction with glass. The glass is deposited in an annular moat, obtained by etching the silicon down to the N- epitaxial layer, a complex and expensive process. The moat, circumscribing each transistor to isolate its base from the other transistors' bases, gives rise to the mesa structure of the transistor. Mesa transistors exhibit low reliability with respect to transistors obtained by planar technologies, in which the base-collector junction is passivated by means of thermal oxide.
In view of the state of the art described, an object of the present invention is to provide a bipolar power transistor obtained with a planar manufacturing process and which has a high base-collector breakdown voltage and has an electric field distribution in the base region Like that of mesa technology.